Don't understand treating copper plated like lead bullets
I see a lot of people saying to treat copper plated bullets like lead when working up a load. But the other day when I ordered some 9mm Ranier copper plated bullets it said to keep the velocity below 1500 fps. Well all of my recipes for 9mm FMJ are under 1500 fps so I would assume I can use these recipes for the copper plated rounds. If I were to load them as I do lead bullets the velocity would be around 900 to 1100 fps. I would think that as long as I adhere to the less than 1500 fps I could use FMJ recipes. What am I missing here?